Recommendations for the Use of $150,000
in the PUC Budget, 2001
Earmarked for Lake Merced
The Lake Merced Task Force
December 14, 2000
Summary
Members of the three working committees of the Lake Merced Task Force (Natural Resources, Recreation, and Water) met to discuss best use of the $150,000 allocated to the Public Utilities Commission, and earmarked for Lake Merced, at the special request of Supervisor Mabel Teng. We recommend the following use of those funds:
Ø Retain the Point Reyes Bird Observatory to develop a Habitat Model of Lake Merced. This model will be used to evaluate the environmental impact of rising lake levels, and to assist in the development of mitigating actions that reduce or eliminate any negative impacts that might otherwise result. ($75,000)The dollar amounts indicated are preliminary estimates, and are intended primarily to suggest that the two program elements being funded are regarded as of equal priority by our committee. We recognize that the amounts indicated may not be sufficient to complete these tasks, and are willing to work with other funding agencies to secure matching funding. We have not allocated funds to the collection of water quality data as we believed these to be a current responsibility of these agencies. However, should additional funding be needed in this area as well we will be happy to consider reallocation of funds to support this need.Ø Retain hydrologists Gus Yates and John Fio to calibrate the Westside Basin Model in order to effectively evaluate the impact of restoration programs as they impact both the Westside Basin Aquifer and Lake Merced. ($75,000)
Ø Incorporate various water quality data collected by the Public Utilities Commission and by other public and private water agencies operating in the Westside Basin to assure that remedial actions taken to preserve the Westside Basin aquifer and to restore Lake Merced do not have a deleterious impact on the beneficial uses of these water resources.
We again emphasize, however, that these activities, however valuable, should not be construed as rationale for delaying such common-sense programs as the use of recycled water for non-potable applications such as irrigation of parks, golf courses and cemeteries.
The Lake Merced Task Force again express our appreciation to Supervisor Teng for her support of Lake Merced, and for securing the funds that make this program a possibility.
The Habitat Model
The Lake Merced Task Force recommends that $75,000 be allocated to retain the Point Reyes Bird Observatory to develop a Habitat Model of Lake Merced. This model will be used to evaluate the environmental impact of rising lake levels, and to assist in the development of mitigating actions that reduce or eliminate any negative impacts that might otherwise result. Activities to be undertaken include:
1. Refine the habitat area curves in Figures 3-5 through 3-7 of the Geo/Resource study by using the Entrix bathymetric data to tabulate the following variables as functions of lake level: 1) linear feet of shoreline perimeter, 2) acres of water surface in various depth ranges, and 3) acres of bank in various steepness ranges. Select the categories based on habitat suitability ranges for fish and wildlife that inhabit the lake. This quantitative analysis would provide a good basis for evaluating lake-level trade-offs.The Westside Basin Hydrology Model2. Convene a meeting of biologists to determine whether listed species are present at the lake (e.g., red-legged frog, western pond turtle). If listed species are present, determine whether maintaining the lake at the targeted level would jeopardize those species and if so what mitigating actions m taken.
3. Confirm or revise the target average lake level of 26 feet (staff gage) recommended in the Geo/Resource study. Select a recommended maximum rate of lake level rise or fall for each month of the year based on agreed-upon thresholds of significance for impacts on wildlife.
The Lake Merced Task Force recommends that $75,000 be allocated to retain hydrologists Gus Yates and John Fio to calibrate the Westside Basin Model in order to effectively evaluate the impact of various remedial programs as they impact both the Westside Basin Aquifer and Lake Merced. Activities to be undertaken include:
1. Critically evaluate results of the Geo/Resources model and CH2M-Hill model for calibration errors that shed light on the reasonableness of the assumptions used in each model. Preliminary evaluation of simulation results and rainfall-versus-lake level trends suggest that some modeling errors may not have been detected by the expert review panel.Water Quality Monitoring2. Activate the Geo/Resources groundwater model using the Groundwater Vistas MODFLOW platform, reconstruct the Geo/Resources calibration simulation, and add the Zone budget module to calculate the simulated water budget for Lake Merced during the calibration period. Compare this budget to the budget produced by the CH2M-Hill model.
3. Activate the CH2M-Hill model and compare the results of both models when identical parameters and input data are used. Complete a sensitivity analysis of variables that affect the Lake Merced water budget. Swap assumptions between models to compare results. This analysis will reveal whether differences in data or differences in assumptions and algorithms underlie the differing model results regarding Lake Merced water budget.
4. Review historical maps of existing gutter and storm drain facilities near Lake Merced to quantify the pre-development and present areas of tributary surface runoff into the lake, an important factor in the water budget of Lake Merced.
5. Evaluate measured recession rates of lake level following the additions of SF-PUC water into the lake in 1979 and 1994 to estimate the leakiness of the lakebed. If the lakebed permeability is low, the recession rate would reflect evaporation as the primary determinant of outflow. If the lakebed is permeable, the recession rate would reflect equilibration of the lake level with the surrounding water table as the primary control on outflow.
6. Conduct a pump test at 4-inch-diameter monitoring wells near the south end of the lake to determine whether groundwater in that area is hydraulically coupled to the lake. An upward break in slope in a semi-log plot of draw-down versus time would indicate the presence of hydraulic coupling. This information would help reconcile differences between the Geo/Resource analysis and the CH2M-Hill analysis.
7. Review historical records at SF-PUC to compile dates of operation and pumping rates for the sunset wells and for irrigation withdrawals from Lake Merced during 1920-1940. Compile monthly rainfall data for that period. Evaluate whether the rapid rise of Lake Merced water level during the late 1930's could have resulted from decreased pumping following construction of the Hetch Hetchy project.
8. Close the culvert between North and South Lakes and monitor the water-level response in both lakes. If rainfall and evaporation are the dominant inflow and outflow, the levels will remain similar. If groundwater through-flow is the dominant inflow and outflow, North Lake will be higher and the elevation difference will reflect the product of the groundwater flow rate and the permeability of the roadbed materials between the lakes. Bracket a range of likely through-flow rates based on estimates of permeability.
Incorporate various water quality data collected by the Public Utilities Commission and by other public and private water agencies operating in the Westside Basin to assure that remedial actions taken to preserve the Westside Basin aquifer and to restore Lake Merced do not have a deleterious impact on the beneficial uses of these water resources.
1. Measure temperature and dissolved oxygen profiles in summer and fall to determine whether the lake stratifies thermally and whether trout habitat is limited to the lower part of the lake during periods of stratification. This information could affect the beneficial use-versus-water level curve for trout.2. Measure turbidity and total coliforms in Vista Grande storm water numerous times throughout the winter at a range of flows to quantify the duration and additional loading associated with first-flush runoff. Prior water quality sampling focused on relatively dirty first-flush runoff. Proposals to increase diversions of Vista Grande water into Lake Merced recommend excluding first-flush runoff from the diversions, but the water quality differences between the first flush and subsequent runoff is not well documented.
3. Evaluate historical turbidity and total coliform levels in Lake Merced in relation to rainfall events, and estimate the percentage of those pollutants that derive from inflow to the lake as opposed to sources within the lake (e.g. waterfowl and algae). These are the two water-quality parameters that most strongly limit emergency potable use of Lake Merced water.
4. Determine whether diversion of Vista Grande water into the lake would alter the ability to use lake water as an emergency potable supply. This would involve a comparison of treatment and Boil-Water-Order requirements with existing lake quality versus the quality with periodic addition of Vista Grande water.
5. Review the current program of waste disposal from the ParkMerced and Stonestown complexes to evaluate the risk of contamination resulting from a rupture of the sewage pipe currently crossing the Impound Lake.
6. Evaluate engineering, permitting, and cost considerations relevant to percolation of treated effluent along the sand barrier between Lake Merced and the ocean. The purpose of such a project would be to develop a groundwater mound that would retard seepage out of the lake.
7. Determine water quality requirements necessary to enable recharge of the Westside Basin Aquifer using tertiary treated water from the Daly City water treatment facility. Assure that this water complies with PGA water quality standards necessary for golf course irrigation.
8. Sample water quality at monitoring wells installed by CH2M-HILL Compile all available water quality data collected by various public and private utilities.
Conclusion
The dollar amounts indicated are preliminary estimates, and are intended primarily to suggest that the two program elements being funded are regarded as of equal priority by our committee. We recognize that the amounts indicated may not be sufficient to complete these tasks, and are willing to work with other funding agencies to secure matching funding. We have not allocated funds to the collection of water quality data as we believed these to be a current responsibility of these agencies. However, should additional funding be needed in this area as well we will be happy to consider reallocation of funds to support this need.
We again emphasize, however, that these activities, however valuable, should not be construed as rationale for delaying such common-sense programs as the use of recycled water for non-potable applications such as irrigation of parks, golf courses and cemeteries.
The Lake Merced Task Force again express our appreciation to Supervisor Teng for her support of Lake Merced, and for securing the funds that make this program a possibility.
The Lake Merced Task Force
December 14, 2000
Participants:
Dick Allen, Golden Gate Heights Neighborhood Association
Tim Colen, Greater West Portal Neighborhood Association
Gary Ehrsam, Dolphin Rowing Club
Dick Morton, SPUR
Dan Murphy, Audubon Society
John Plummer, Friends of Lake Merced
Howard Strassner, Sierra Club
Bud Wilson, West of Twin Peaks Central Council
Technical Advisors:
David Dawdy, Hydrologist, USGS retired
John Fio, Hydrologist, HydroFocus, Inc.
Gus Yates, Hydrologist, private practice
